1. Field of the Invention
The present invention concerns a method for operating a gas turbine group.
2. Discussion of Background
In a thermal power station installation, in particular an air reservoir installation, particular problems can appear with respect to the turbine cooling. These arise from the very high pressure ratio of about 30:1 to 70:1 necessary for compatibility with an economically tolerable compressed air reservoir. This high pressure ratio demands at least one reheat stage in the turbine expansion, again for economic reasons. There is a substantially smaller heat or temperature drop for the high-pressure turbine, referred to below as the HP turbine, than there is for the low-pressure turbine, referred to below as the LP turbine. In an air reservoir gas turbine, in which the compressor is equipped with intercooling, compressed air is delivered to a reservoir cavern by an electrical machine operating as a motor. An air/water heat exchanger cools the heated compressed air coming from the last compressor unit for the purpose of reducing its specific volume and transfers the heat into a hydraulic accumulator installation, the corresponding shut-off units being open during this so-called charging operation. If electrical energy has to be generated again from the stored compressed air, the compressor group is shut down by means of a control system. The turbine group, comprising, for example, an HP turbine, an LP turbine and the electrical machine, which now has to be operated as a generator, is started by opening the corresponding shut-off unit. This only takes place initially by means of compressed air from the reservoir, the compressed air being preheated in the heat exchanger by the stored hot water. After ignition in the combustion chamber, the production of electrical energy can be undertaken. It is, however, also possible to operate the installation as a through-connected gas turbine by a corresponding arrangement of the shut-off units and couplings. It is then simultaneously possible to store some air or to extract it from the cavern. Such an installation can no longer meet the present-day economic specifications with respect to reducing the fuel consumption and an additional consideration is that such installations are not able to satisfy the present-day maximum pollutant emission figures which have to be demonstrated. It is, indeed, correct that an exhaust heat boiler can be added to generate steam. The steam generated in this way can be supplied to the steam turbine group or be supplied directly to the gas turbine group in ways which are known. The question of whether one or the other method of using the steam is preferred depends on the operational period in turbine operation. For less than about 2-4 hours per day, steam injection will be correct. If such a thermal power station installation is now optimized, the hot gas temperatures at the HP turbine inlet corresponding to the state of the art involve such a high outlet temperature of the combustion gases from the turbine that the latter cannot be used directly as the coolant for the LP turbine. For this reason, the turbine is generally cooled with cooling air which is extracted before the cavern of the air reservoir installation, as is usual for cooling turbines. In through-connected gas turbine operation, the cooling air leading to the turbine could, of course, be extracted at a compressor position with matching pressure in order to avoid throttling losses. Whereas the consumption of cooling air for the turbine mainly reduces the turbine efficiency because of mixing losses (for a given mixed temperature at its inlet), the consumption of cooling air for the LP turbine has a very disadvantageous effect because this cooling air bypasses the HP turbine, i.e. it performs no work there.